JPH09199988A - Active filter circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive active filter circuit with a simple configuration in which operation at a high frequency is attained and suitable for general use. SOLUTION: This active filter circuit has 5 filter stages in cascode connection between a power supply and ground. The circuit has two major current paths, a 1st current passes through collector-emitter paths of transistors(TRs) 31, 33, 35, 37, 39 and a 2nd current passes through collector-emitter paths of transistors(TRs) 32, 34, 36, 38, 40. Each major current path is driven by power supplies 61, 62. A differential input signal is applied to base electrodes of the TRs 31, 32 in the operation and a differential output signal is extracted from emitters of the TRs 39, 40. Collectors and base electrodes of the TRs 33, 34 of a stage 71 are interconnected. The TRs 35 to 40 of stages 72 to 74 are similarly connected. In other cases, the cut-off frequency is changed under the control of a controllable power supply.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active filter, and more particularly to an active filter circuit with differential input / output.

[0002]

2. Description of the Prior Art Active filter circuits in the prior art usually require the use of complex circuit elements such as operational amplifiers or transconductance amplifiers. With the use of such a composite element in a differential filter,
Severe restrictions on the frequency variation of the filter may be imposed, but may also require the use of additional common mode ballast circuits.

[0003]

There has been a need for an active filter circuit which can be operated at high frequencies and which is suitable for uniform implementation and which has a low cost and a simple structure.

[0004]

According to the present invention, there is provided an active filter circuit having a plurality of stages, each stage including a first transistor and a second transistor. The first transistor of each of the plurality of stages includes a first transistor
Has a main current path connected to the second series path, and each second transistor has a main current path connected to the second series path. At least one of the plurality of stages
The first and second transistors in the stage have their base and collector electrodes interconnected.

[0005]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

Referring to the drawings, FIG. 1 illustrates a stacked five pass low pass filter circuit according to the present invention. The filter circuit 1 has five cascoded stages and the differential output of one stage is used as the differential input of the following stage. There are two main current paths, the first main current path is the transistors 31, 33, 3
Through the collector-emitter path of 5, 37, 39
The main current paths of the transistors are transistors 32, 34, 36, 38,
40 collector-emitter paths.

The first stage 70 of the filter circuit is np
It has n-transistors 31 and 32, resistors 13 and 14, and a capacitor 51. The first stage 70 has a resistor 11
And 12 are separated from the voltage supply of the filter circuit. The filter circuit input is acted on the base electrodes of the transistors 31 and 32, and the first stage 70 of the filter circuit
There is no interconnection with. The differential output of the first stage 70 is
This is the voltage on the capacitor 51 and the output of this first stage forms the differential input of the second stage 71. Each input signal of the second stage 71 is interconnected to the base electrode of the npn transistor 33 or 34 which controls the other main current path. Transistors 33 and 34 act as voltage followers, allowing the use of higher frequency signals than can be operated by many conventional active filter circuits.

The third, fourth and final stages 72, 73, 74 of the filter circuit, which terminate in capacitors 53, 54, 55 respectively, have a structure similar to that of the second stage 71. The output of the final stage 74 is the power supply 61.
And 62 to ground. The low pass output of the filter circuit is taken through the capacitor 55 in the final stage 74. If desired, the wide pass output can be taken through the collectors of transistors 31 and 32.

Each stage 70-74 of the circuit has a first transfer function. The five stage circuit 1 will therefore have a fifth transfer function. From this,
The frequency and phase characteristics of the filter can be calculated.

By the network conversion described below,
The values of the elements of the filter circuit according to the present invention are
It can be obtained from the values of the elements of the C filter circuit. This, together with reference to FIGS. 4, 5, 6, and 7, helps to understand the present invention.

FIG. 4 shows a basic LRC filter having a power supply drive 41 and a 1 ohm terminal resistor 42. If the resistances of 1 ohm are connected in parallel but the combined resistance is not infinite, the addition of such a parallel resistance from the end of each coil L2, L4 to ground does nothing for the overall response in the filter. Seems to have no effect. The circuit resulting from the loading of these resistors 43-48 is shown in Figure 4a. The coil L2 of this circuit,
L4 can now be replaced with a capacitor that uses a triangular star transformation. This transformation is shown in the Laplace region in Figure 4b. The resistors 48 and 49 at the terminal part in FIG. 4a can be replaced by equivalent open circuit terminals. The power source 41 and the parallel resistor 43 are connected to the series resistor 40 in FIG.
2 can be replaced by an equivalent voltage source 401. It will be appreciated that the resulting circuit in Figure 4c has only positive and negative value resistors and capacitors.

In such a conversion, the requirement for a negative value component is a negative impedance converter (N
ICs) can be removed. These are ideal circuit elements, effectively providing the current negative impedance at the output seen by the input. By loading NICs through the network in FIG. 4c, negative value components can be eliminated. This is shown in Figure 4d.

NICs 411-414 in FIG. 4d
Each can be replaced by a single increasing voltage follower with a load sensing current feedback, which occurs in the circuit in FIG. Implementation of these voltage followers 415-418 with current biased bipolar transistors results in the filter circuit in FIG. 1, where:
The interconnected differential structure provides the necessary negative current feedback due to the voltage followers. This structure
We can think in terms of two half circuits. By one is + V _in, the other one is driven by -V _in. The current and voltage in one half circuit will be the inverse of the current and voltage in the other half circuit. These half-circuits can correspond to emitter-follower circuits, each with a constant power supply from the emitter electrode to ground, series resistance, and capacitor. This series resistance is from the emitter electrode to the respective output and the capacitor is from the output to ground. These two half circuits are interconnected to provide the necessary feedback shown in FIG.

In two stacked filter stages of this type, shown in FIG. 7, current is saved. This is because the subsequent filter stage operates at the bias current used to drive the lowest pass filter stage. Efficient design of this current is of utmost importance. Power supply is limited there. For example, batteries powered by fixed and fluid applications.

By connecting the capacitors in FIG. 7 differentially, more compact filter circuits may be obtained instead of showing a single-ended arrangement.

The above results equally appear in two of the five stage filter circuits shown in FIG.

As will be appreciated, the capacitors need not be connected differentially. However, as shown in FIG. 7, one end on each side of the stack needs to be connected to ground. This is particularly useful for uniform implementations, where fully floating capacitors are not supported.

Higher operational frequencies are possible by involving only emitter followers in the circuit design and avoiding the use of devices such as operational amplifiers and transconductance amplifiers.

The operation of only the AC feedback mechanism within the circuit renders the filter much less sensitive to DC offset than prior art filters.

The use of a transistor as a voltage follower, rather than using a transconductance amplifier or the like, ensures that the dc gain of the filter as found at the low pass output is unity. To do.

The fact that the common voltage at each node is well defined eliminates the need for an additional common mode ballast circuit, in contrast to the existence of a transconductor / capacitor filter implementation.

Due to the compatibility between the conventional LCR filter and that of the present invention, the component values required to implement the filter can be easily calculated.

The layered array of active filters in the present invention is not limited to use in high and low pass filters, as shown in FIG. 1, but is equally applicable to all other filter types. .

FIG. 2 is an example of a third elliptical filter implemented with interconnected bipolar transistors. The elements of the circuit are essentially similar, except for the addition of capacitors 101 and 102, and are connected to show that the filter gets zero at a finite frequency.

FIG. 3 shows an example of how the third all-pass filter is constructed. Capacitor 201,
202, 203, 204 provide the filter with dependent phase adjustment features for its frequency.

FIG. 3 has also shown that semiconductor devices other than npn bipolar transistors can be used in filter implementations. Field effect transistors 211-216 are used in place of npn transistors. Operated in the same way under the control of a voltage follower, the connected power supply and drain electrode are matched to the base, emitter and collector electrodes of the bipolar transistor.

FIG. 3 also illustrates how the cutoff frequency of a filter constructed in the present invention can be controlled.
The mounted resistance appearing in the filters in FIGS. 1 and 2 is removed and the circuit operates itself due to the use of the dynamic output impedance of the transistor. Since this impedance is a function of the DC bias current, the cutoff frequency of the filter fluctuates under the control of the power supplies 221 and 222.

[0028]

As will be appreciated by those skilled in the art, the differential nature of the circuit provides increased noise immunity and linearity. The filter circuit in the present invention is equally applicable to discontinuous and uniform implementations.

[Brief description of drawings]

FIG. 1 illustrates a stacked low pass filter in the present invention.

FIG. 2 illustrates a stacked elliptical filter according to the present invention.

FIG. 3 illustrates a tunable all-pass stacked filter in the present invention.

FIG. 4 is a diagram showing steps for network conversion.

FIG. 5 is a diagram showing steps for network conversion.

FIG. 6 is a diagram showing steps for network conversion.

FIG. 7 is a diagram showing steps for network conversion.

[Explanation of symbols]

1 5 stage circuit 11, 12, 13, 14 resistance 31, 32 transistor 33, 34 npn transistor 35, 36, 37, 38, 39, 40 transistor 41 power supply drive 42 terminal resistance 43 parallel resistance 44, 45, 46, 47 , 48, 49 resistors 51, 53, 54, 55 capacitors 61, 62 power source 70 first stage 71 second stage 72 third stage 73 fourth stage 74 final stage 101, 102, 201, 202, 203, 204 capacitors 211, 212, 213, 214, 215, 216 Transistor 221, 222 Power supply 401 Voltage source 402 Series resistance 411, 412, 413, 414 NICs 415, 416, 417, 418 Voltage follower

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[Procedure amendment]

[Submission date] December 24, 1996

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] All figures

[Correction method] Change

[Correction contents]

[Fig. 2]

[Figure 3]

FIG.

FIG. 4

[Figure 5]

FIG. 6

FIG. 7

Claims (10)

[Claims] 1. Each stage has a plurality of stages including first and second transistors, each of the first transistors of the plurality of stages being connected to a first series path. A main current path, each second transistor of the plurality of stages has a main current path connected to a second series path, and at least one stage of the plurality of stages The first and second transistors are active filter circuits having their base and collector electrodes interconnected. 2. The first stage of the plurality of stages comprises the filter circuit input stage, and the differential input signal is the first and second stages of the first stage.
2. The active filter circuit according to claim 1, wherein the collector / source electrodes of the first and second transistors of the first stage are connected to a voltage source, the collector / electrode being applied to a base electrode of the transistor. 3. The active filter circuit according to claim 2, wherein the collector electrodes of the first and second transistors are connected to the voltage source via each resistance element. 4. One final stage of the plurality of stages comprises the filter circuit output stage, and the emitter electrodes of the first and second transistors of the final stage are routed through respective power supplies. 4. The active filter circuit according to claim 1, wherein the active filter circuit is connected to a ground potential and each output terminal of the filter circuit. 5. The emitter electrode of the first and second transistors of the final stage is connected to the power supply and the output of the filter circuit via each resistance element. The active filter circuit described. 6. The active filter circuit of claim 4, wherein the power supply is an electronically controllable current source and is varied to change the characteristics of the filter. 7. The active filter circuit according to claim 6, wherein one of the characteristics of the filter is the cutoff frequency. 8. The active filter circuit according to claim 1, wherein at least one stage of the filter stages is terminated by differentially connected capacitors. 9. The filter stage according to claim 1, wherein at least one of the filter stages has emitter electrodes of both the first and second transistors connected to a ground potential by a capacitor. The active filter circuit according to. 10. At least one of the stages further includes a resistance element, and the resistance element includes an emitter electrode in each of the first and second transistors of the at least one stage and the resistance element. 10. The active filter according to claim 8, wherein the active filter is connected between capacitors.